The present invention relates to automatic gain control systems and more particularly to a system having both analog and digital gain stages controlled by a digital control system.
Signal transmission systems, e.g. broadcast or cable television systems, generally transmit radio frequency signals over long distances. The signal levels at various receiving locations will vary due to differing transmission distances and various levels of attenuation which may vary from time to time. The receiving systems generally work best if the received signal is at some optimum amplitude level for processing the signals. It is common practice to use automatic gain control, AGC, systems for amplifying received signals to an optimum level for the signal processing systems. An AGC system generally includes a variable gain amplifier and a controller which compares the amplified signal level to a desired level and adjusts the amplifier gain as needed.
While transmitted signals are analog, it is now common for most signal processing to be performed by digital systems, e.g. digital signal processors. After a transmitted signal is received, it may be converted to a digital signal by an analog to digital, A/D, converter. It is desirable that the received analog signal be amplified before the signal is coupled to the AND converter to allow use of the full range of operation of the A/D converter. Use of the full range provides the best signal to noise ratio for the system.
Digital gain units, e.g. multipliers, may also be used to control signal amplitude once the received signal has been digitized. Digital multipliers are generally more cost effective than digitally controlled analog amplifiers in terms of the amount of board area or silicon area required to implement an AGC with a given number of gain settings. From this point of view, efficiency can be obtained by performing gain control with a digital multiplier and having a fixed gain analog amplifier driving the A/D. However, the tradeoff is the reduction in signal to noise ratio which results from not using the full operating range of the A/D.
It would be desirable to have an automatic gain control system which combines benefits of digital gain units with the improved signal to noise ratio which is achieved by use of an gain controlled analog amplifier driving the input to an A/D.